Pretty sure they just choose to, if they pay the bonus as regular income in your paycheck, or as a separate income,it hits the flat withholding.
I haven’t been an employer for a couple decades now, but don’t recall that option at all. I see more details on it here:
https://turbotax.intuit.com/tax-tips/jobs-and-career/how-bonuses-are-taxed/L7UjtAZbh